As a method for detecting a state of charge (SOC) of a storage battery, for example, as described in Japanese Unexamined Patent Application Publication No. 2004-530880, there is a method for estimating SOC by measuring the battery voltage with using the relation (linearity) between a battery voltage and SOC.
In this method, it is not possible to measure a stabilized battery voltage because the battery voltage is greatly affected by the over-voltage due to charge or discharge. Particularly, in the vehicle, the storage battery supplies a power to each load during and after engine start and is charged by a charger as well, thereby being charged and discharged repeatedly. Accordingly, the stabilized battery voltage can not be obtained, resulting in generation of significant errors in estimation of the SOC obtained from the linearity.
Therefore, for a storage battery mounted on the vehicle, particularly, a storage battery supplying a power to a load only at emergency without regularly being connected to the load, there is a method for estimating SOC by substituting a measured data of battery voltage into a linear expression after measuring the battery voltage during the period between an door-open and engine startup, or a method for estimating SOC by periodically detecting battery voltages for long hours with a timer to obtain stabilized voltage values. The methods described above are lack of accuracy, because the methods utilize the measured raw data for the estimation and there is no consideration of, for example, a state of health (SOH).
Besides, methods for estimating an SOC of a storage battery during being charged includes a method in which a charged amount is calculated by multiplying the current value by hour, followed by comparing the charged amount with battery capacity and normalizing data.
However, the battery capacity of the storage battery is lowered and decreased from its initial capacity in accordance with the deterioration of the storage battery, thus the accuracy of the calculated SOC becomes lower than that of the initial stage associated with the deterioration. Specially, this tendency appears outstandingly in the case when an SOC is low due to, for example, the discharge of the battery. As measures against this problem, there has been known methods for correcting a battery capacity based on the deterioration level as described in Japanese Patent Application Publications No. H6-59003 and No. 2000-166109.
The methods for correcting a battery capacity based on a deterioration level described in Japanese Patent Application Publications No. H6-59003 and No. 2000-166109 include: estimating a residual charge at the full charged (SOC: 100%) based on the residual charge at heavy loaded obtained by using the IV method which calculates the relation between the discharge current of the storage battery and the battery voltage at every SOC; calculating a deterioration level by dividing the residual charge by the battery capacity; and correcting the battery capacity in accordance with the deterioration level.
These methods also use only the SOC at the time when measured to determine an SOC and thus lack the accuracy of the determination.
There are methods for detecting an SOH such as a method for detecting an SOH based on an increase of an internal resistance of a battery, a method for detecting an SOH from a voltage when a storage battery is discharged, and a method for detecting an SOH by estimating a capacity of a storage battery from a voltage when the battery is discharged. Any of the methods use only an SOH at the time when measured without consideration of factors, like temperature or other and thus lack the accuracy of the determination of an SOH.
Technology for determining an SOH of a secondary battery such as a lead battery mounted on the vehicle and the like is described in Japanese Patent Application Publication No. 2001-228226.
Generally, since there is a strong correlation between an internal impedance or an internal resistance and an SOH of a secondary battery, if an internal impedance or an internal resistance of a secondary battery can be obtained, it is possible to determine a deterioration level of the secondary battery from the obtained result. This makes it possible to inform the user of the need for replacing the deteriorated battery.
In order to determine an SOH of the secondary battery in a power supply system having a secondary battery, it may be employed a construction in which the secondary battery is charged or discharged with a specified current, a current and a voltage at that time are measured, and an internal impedance or an internal resistance is calculated from the measured current and voltage by using a specified operation.
In the case when a secondary battery is mounted on the vehicle and the like, it is important that normal operation of the secondary battery be guaranteed over a very wide range of operating temperature in supposing the usage in various areas and environments.
Meanwhile, the internal impedance or internal resistance of the secondary battery has a tendency to be changed a great deal depending on temperature and to be significantly increased specially at low temperature. Accordingly, even if the internal impedance or internal resistance falls within the allowable range at normal temperature, they may be out of the allowable range at lower temperature, and thus it may not be able to use the secondary battery under the lower temperature.
Consequently, it becomes important to accurately determine the SOH regardless of the operating temperature of the second battery, thus it is required that the internal impedance or internal resistance should be estimated after temperature correction by means of any method. The internal impedance or internal resistance of the secondary battery has a complicated temperature characteristic, so it is difficult to make an accurate approximation with a simple approximate formula, and it has been difficult to make temperature correction for the internal impedance or internal resistance with high accuracy.
Japanese Patent Application Publication No. 2005-091217 discloses the related technology for determining SOH of a secondary battery with high accuracy by correcting a temperature characteristic of an internal impedance with use of a polynomial expression of at least the third or more degree.
Generally, the impedance of the battery has a strong correlation with the deterioration level or discharge capability of the battery, thus, if the impedance of the battery can be obtained, the deterioration level or discharge capability of the battery can be estimated by the obtained impedance. This makes it possible to inform the user of the need for replacing the deteriorated battery or the battery having lowered discharge capability.
In order to estimate a deterioration level or discharge capability of a secondary battery in a power supply system having the secondary battery, there has been conventionally known a method which includes: letting a specified charge current or discharge current flow to the battery; measuring the current and the voltage at the time; and calculating the internal impedance by a specified operation from the measured current and voltage.
The impedance of the battery can be an index for showing an accurate deterioration level or discharge capability of the battery if measurements are carried out under the same conditions. However, in the case where the impedance measurement is applied to a practical system, specially to the vehicle as a typical example, a charge current and a discharge current such as a charge current regularly flowing from an alternator to the battery and a discharge current to be supplied to each electric device flow through the battery mounted on the vehicle during a normal operation.
Thus, the impedance measurement has been performed in the state that the charge current and the discharge current during the normal operation are superimposed with or added to the specified charge current or the discharge current described above, without considering the difference of the measuring condition due to the superimposed charge/discharge current.